1. Field of Invention
The present invention relates generally to a method and an apparatus for reinforcing columns and tanks such that the columns and tanks are better able to resist torsional forces and bending forces, and more particularly to a method and apparatus for confining columns and tanks.
2. Description of the Prior Art
Prior to the year 1971, concrete columns used to support bridges and overpasses, like those used to support overpasses over freeways, were generally not specifically built to withstand seismic events, e.g. earthquakes. A seismic event may result in the buckling of a vertical column which has not been designed to withstand the seismic event.
Concrete columns built in 1971 and before generally included vertical steel bars which were intended to reinforce the columns. However, the columns did not typically have enough hoop reinforcement to prevent buckling of the columns during earthquakes. While the concrete columns reinforced with vertical steel bars were strong enough to support superstructures like bridges, the concrete columns were often not strong enough to sustain the torsional forces and bending forces which occur during seismic events.
The San Fernando earthquake, which occurred in California in 1971, caused the failure, e.g. buckling, of a number of concrete columns due to insufficient hoop reinforcement. The columns buckled when the vertical steel bars would break loose from the concrete, especially near the base and the top of the columns, i.e. near the foundation and near the superstructure. Hence, in an effort to prevent buckling of concrete columns during earthquakes and other seismic events, since 1971, spiral reinforcement of the vertical bars in newly built concrete columns has been strongly recommended and, in some geographic areas, required. Further, a large number of concrete columns built prior to 1971 are retrofitted to provide them with sufficient hoop reinforcement, e.g., confinement of longitudinal reinforcement, to better survive seismic events.
Conventional methods used to reinforce concrete columns, particularly those built prior to 1971, include tightly, closely wrapping the column with wire to reinforce the column using transverse post tensioning, and confining the column with a steel sheet. While these methods tend to be effective in providing hoop reinforcement for concrete columns and similar structures, as for example concrete tanks, they tend to be aesthetically unpleasing and costly, as completely covering a concrete column with wire or steel can be expensive. Further, the amount of active confinement of a column in relation to the amount of wire or steel used shows that the conventional methods could be more efficient. That is, the conventional methods do not place the columns in compression, which would serve to more strongly reinforce the columns by rendering the columns to be more resistant to torsional forces and bending forces, although the columns are covered with wire or steel. As such, what is desired is a method and an apparatus for efficiently retrofitting concrete columns and similar structures which would provide an improved amount of active confinement for a given amount of steel.